Flash EEPROM system with simultaneous multiple data sector programming and storage of physical block characteristics in other designated blocks

ABSTRACT

A non-volatile memory system is formed of floating gate memory cells arranged in blocks as the smallest unit of memory cells that are erasable together. The system includes a number of features that may be implemented individually or in various cooperative combinations. One feature is the storage in separate blocks of the characteristics of a large number of blocks of cells in which user data is stored. These characteristics for user data blocks being accessed may, during operation of the memory system by its controller, be stored in a random access memory for ease of access and updating. According to another feature, multiple sectors of user data are stored at one time by alternately streaming chunks of data from the sectors to multiple memory blocks. Bytes of data in the stream may be shifted to avoid defective locations in the memory such as bad columns. Error correction codes may also be generated from the streaming data with a single generation circuit for the multiple sectors of data. The stream of data may further be transformed in order to tend to even out the wear among the blocks of memory. Yet another feature, for memory systems having multiple memory integrated circuit chips, provides a single system record that includes the capacity of each of the chips and assigned contiguous logical address ranges of user data blocks within the chips which the memory controller accesses when addressing a block, making it easier to manufacture a memory system with memory chips having different capacities. A typical form of the memory system is as a card that is removably connectable with a host system but may alternatively be implemented in a memory embedded in a host system. The memory cells may be operated with multiple states in order to store more than one bit of data per cell.

BACKGROUND OF THE INVENTION

[0001] This invention relates to semiconductor memory systems,particularly to non-volatile memory systems, and have application toflash electrically-erasable and programmable read-only memories(EEPROMs).

[0002] Flash EEPROM systems are being applied to a number ofapplications, particularly when packaged in an enclosed card that isremovably connected with a host system. Current commercial memory cardformats include that of the Personal Computer Memory Card InternationalAssociation (PCMCIA), CompactFlash (CF), MultiMediaCard (MMC) and SecureDigital (SD). One supplier of these cards is SanDisk Corporation,assignee of this application. Host systems with which such cards areused include personal computers, notebook computers, hand held computingdevices, cameras, audio reproducing devices, and the like. Flash EEPROMsystems are also utilized as bulk mass storage embedded in host systems.

[0003] Such non-volatile memory systems include an array offloating-gate memory cells and a system controller. The controllermanages communication with the host system and operation of the memorycell array to store and retrieve user data. The memory cells are groupedtogether into blocks of cells, a block of cells being the smallestgrouping of cells that are simultaneously erasable. Prior to writingdata into one or more blocks of cells, those blocks of cells are erased.User data are typically transferred between the host and memory array insectors. A sector of user data can be any amount that is convenient tohandle, preferably less than the capacity of the memory block, oftenbeing equal to the standard disk drive sector size, 512 bytes. In onecommercial architecture, the memory system block is sized to store onesector of user data plus overhead data, the overhead data includinginformation such as an error correction code (ECC) for the user datastored in the block, a history of use of the block, defects and otherphysical information of the memory cell block. Various implementationsof this type of non-volatile memory system are described in thefollowing United States patents and pending applications assigned toSanDisk Corporation, each of which is incorporated herein in itsentirety by this reference: U.S. Pat. Nos. 5,172,338, 5,602,987,5,315,541, 5,200,959, 5,270,979, 5,428,621, 5,663,901, 5,532,962,5,430,859 and 5,712,180, and application Ser. No. 08/910,947, filed Aug.7, 1997, and Ser. No. 09/343,328, filed Jun. 30, 1999. Another type ofnon-volatile memory system utilizes a larger memory cell block size thatstores multiple sectors of user data.

[0004] One architecture of the memory cell array conveniently forms ablock from one or two rows of memory cells that are within a sub-arrayor other unit of cells and which share a common erase gate. U.S. Pat.Nos. 5,677,872 and 5,712,179 of SanDisk Corporation, which areincorporated herein in their entirety, give examples of thisarchitecture. Although it is currently most common to store one bit ofdata in each floating gate cell by defining only two programmedthreshold levels, the trend is to store more than one bit of data ineach cell by establishing more than two floating-gate transistorthreshold ranges. A memory system that stores two bits of data perfloating gate (four threshold level ranges or states) is currentlyavailable, with three bits per cell (eight threshold level ranges orstates) and four bits per cell (sixteen threshold level ranges) beingcontemplated for future systems. Of course, the number of memory cellsrequired to store a sector of data goes down as the number of bitsstored in each cell goes up. This trend, combined with a scaling of thearray resulting from improvements in cell structure and generalsemiconductor processing, makes it practical to form a memory cell blockin a segmented portion of a row of cells. The block structure can alsobe formed to enable selection of operation of each of the memory cellsin two states (one data bit per cell) or in some multiple such as fourstates (two data bits per cell), as described in SanDisk CorporationU.S. Pat. No. 5,930,167, which is incorporated herein in its entirety bythis reference.

[0005] Since the programming of data into floating-gate memory cells cantake significant amounts of time, a large number of memory cells in arow are typically programmed at the same time. But increases in thisparallelism causes increased power requirements and potentialdisturbances of charges of adjacent cells or interaction between them.U.S. Pat. No. 5,890,192 of SanDisk Corporation, which is incorporatedherein in its entirety, describes a system that minimizes these effectsby simultaneously programming multiple chunks of data into differentblocks of cells located in different operational memory cell units(sub-arrays).

SUMMARY OF THE INVENTION

[0006] There are several different aspects of the present invention thatprovide improvements in solid state memory systems, including thosedescribed above. Each of these aspects of the present invention, themajor ones being generally and briefly summarized in the followingparagraphs, may be implemented individually or in various combinations.

[0007] Multiple user data sectors are programmed into a like number ofmemory blocks located in different units or sub-arrays of the memoryarray by alternately streaming data from one of the multiple sectors ata time into the array until a chunk of data is accumulated for each ofmultiple data sectors, after which the chunks are simultaneously andindividually stored in respective blocks in different units of thememory. This increases the number of memory cells that may be programmedin parallel without adverse effects.

[0008] An error correction code (ECC), or other type of redundancy code,may be generated by the controller from the streaming user data duringprogramming and written into the same memory block as the user data fromwhich it is generated. The redundancy code is then evaluated by thecontroller when the sector of data is read out of the memory block. Asingle redundancy code generation circuit is utilized, even when thestreaming data is alternated between data chunks of the multiplesectors, by providing a separate storage element for each of the userdata sectors being programmed at the same time, in which intermediateresults of the generation are temporarily stored for each sector.

[0009] Overhead data of the condition, characteristics, status, and thelike, of the individual blocks are stored together in other blocksprovided in the array for this purpose. Each overhead data record mayinclude an indication of how many times the block has been programmedand erased, voltage levels to be used for programming and/or erasing theblock, whether the block is defective or not, and, if so, an address ofa substitute good block, and the like: A group of blocks are devoted tostoring such records. A large number of such records are stored in eachof these overhead blocks. When accessing a specific user data block toperform one or all of programming, reading or erasing, the overheadrecord for that user data block is first read and its information usedin accessing the block. By storing a block's overhead data outside ofthat block, frequent rewriting of the overhead data, each time the userdata is rewritten into the block, is avoided. It also reduces the amountof time necessary to access and read the block overhead data when theblock is being accessed to read or write user data. Further, only oneECC, or other redundancy code, need be generated for the large number ofoverhead records that are stored in this way.

[0010] The records from a number of overhead blocks can be read by thecontroller into an available portion of its random-access memory forease of use, with those overhead blocks whose records have not beenaccessed for a time being replaced by more active overhead blocks in acache-like manner. When a beginning address and number of sectors ofdata to be transferred is received by the memory controller from thehost system, a logical address of the first memory block which is to beaccessed is calculated in order to access the overhead record for thatblock but thereafter the overhead records are accessed in order withouthaving to make a calculation of each of their addresses. This increasesthe speed of accessing a number of blocks.

[0011] Information of defects in the memory, such as those discoveredduring the manufacturing process, may also be stored in separate blocksdevoted for this purpose and used by the controller so that theimperfect memory circuit chips may be included in the memory systemrather than discarding them. This is particularly an advantage when asingle defect record affects many blocks. One such defect is a badcolumn that is shared by a large number of blocks. A number of badcolumn pointers (BCPs) may be stored together as a table in one or moresectors that are devoted in part or entirely to this overhead data. Whenthis is done, the physical location of the streaming user data beingwritten to the memory is shifted when a comparison of the relativephysical location within a sector of individual bytes of data with theBCP table indicates that the data byte is being directed to at least onememory cell that is along a bad column. The reverse is performed duringreading, where data bits read from memory cells that were skipped overduring write because of a bad column are ignored.

[0012] Since flash EEPROM cells have, by their nature, a limited life interms of the number of times that they can be erased and reprogrammed,it is usually prudent to include one or more operational features thattend to even out the wear on the various memory blocks that can becaused by multiple rewrites to the same blocks. One such techniquealters from time-to-time the correspondence between the digital data andthe memory states that are designated to represent the digital data. Toaccomplish this in the present memory system, the first one or few bitsof the initial byte of the individual sectors of data, termed herein asa flag byte, are used to designate such correspondence. These bits aredesignated upon writing user data, and all data following the initialbits are transformed on the fly as the streaming data is beingtransferred to the memory array in accordance with their value. Uponreading a sector of data, these initial bit(s) are read and used totransform back all subsequent data stored in the sector to theiroriginal values as the data are being read out of the memory in astream.

[0013] When the memory system is formed of multiple memory cell arrays,such as by using two or more integrated circuit chips that each includesuch an array, the system's manufacture and use is simplified byaccumulating information about each of the memory arrays in the system,and then storing that information as a single record in some convenientlocation, such as in one of the blocks of one of the memory arrays. Thismakes it much easier to combine memory arrays having different sizesand/or operating characteristics into a single system. One such recordmerges the number of blocks of user data available in each of the memorychips in a way that establishes a continuum of logical block addressesof the blocks of all the arrays in the system. When a location of memoryis being accessed for a read or write operation, the memory controllerthen accesses the merged record in a process of converting a logicalblock address to a physical address of a block in one of the memoryarrays.

[0014] Such a merged record can be automatically generated and storedduring manufacturing by the controller reading the information from eachof the memory arrays, merging that information into a single record, andthen writing that record into a designated block of one of the memoryarrays. Currently, the memory controller is usually provided on aseparate integrated circuit chip, with one or more memory cell arraychips connected with the controller. It is contemplated, in light ofcontinuing processing technology improvements, that a memory array canalso be included on the controller chip. When that amount of memory isinsufficient for a particular system, one or more additional circuitchips containing further memory array(s) are then utilized. When two ormore physically separate arrays of included in a system, then thegeneration of the merged record simplifies operation of the controllerto address blocks across multiple arrays.

[0015] Other characteristics of various memory array circuit chips usedto form a system, such as optimal voltage ranges, timing, numbers ofpulses used, characteristics of voltage pumps, locations of overheadblocks, and the like, can vary without adverse effects. These operatingcharacteristics can also be tabulated into a single system file foraccess by the micro-controller, or, more conveniently, themicro-controller can be operated to first access those of suchcharacteristics as are necessary from an individual memory array chipbefore accessing that chip to read or write data. In either case, thisallows the memory system to be formed with memory chips having differentcharacteristics without any degradation in its performance. Themanufacturing of non-volatile memory systems is simplified since all thememory array chips in a system need not be the selected to be the same.

[0016] Additional aspects, features and advantages of the presentinvention are included in the following description of specificembodiments, which description should be taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a block schematic diagram of an example non-volatilememory system in which various aspects of the present invention may beimplemented;

[0018]FIG. 2 is a more detailed schematic diagram of one of the memorycell array units, with associated logic and buffers, of the system ofFIG. 1;

[0019]FIGS. 3A and 3B illustrate four state and two state operation,respectively, of the individual memory cells of the system of FIG. 1;

[0020]FIG. 4 is an example of the content and structure of data storedin one of the memory blocks of the system of FIG. 1;

[0021]FIG. 5 illustrates streaming data transfer during programming ofmultiple sectors of data at one time within the memory system of FIG. 1;

[0022]FIG. 6A is a schematic block diagram of the circuit of the systemof FIG. 1 that utilizes bad column pointers (BCP) and generates an errorcorrection code (ECC), or other data redundancy code, with themulti-sector streaming data being transferred to the memory cell array;

[0023]FIG. 6B is a schematic diagram of the ECC generation block of FIG.6A;

[0024]FIG. 6C is a schematic diagram of the BCP processing block of FIG.6A;

[0025]FIG. 7 illustrates the manner in which data from multiple sectorsis transferred to the memory cell array;

[0026]FIG. 8 illustrates an example data structure of block overheaddata records;

[0027]FIG. 9 shows an example of an overhead data record of FIG. 8 for agood memory cell block;

[0028]FIG. 10 shows an example of an overhead data record of FIG. 8 fora defective memory cell block;

[0029]FIG. 11 illustrates an example data structure of a reserved blockthat stores records of defects of the memory cell array, namely thelocation of bad columns, of the system of FIG. 1;

[0030]FIG. 12 shows an example of the physical partitioning of thememory array of FIG. 1 into units and blocks of memory cells, and thedata that is designated to be stored in them;

[0031]FIG. 13 schematically illustrates a method of forming a mergedrecord of the characteristics of multiple memory integrated circuitchips of the system of FIG. 1;

[0032]FIG. 14 shows an example of a merged record formed in the mannerillustrated in FIG. 13;

[0033]FIG. 15 illustrates an example of bit fields of the flag byte ofthe data sector of FIG. 4;

[0034]FIG. 16 schematically shows generation and use of the bit fieldsof the flag byte of FIG. 15 during writing of data in the memory; and

[0035]FIG. 17 schematically shows use of the bit fields of the flag byteof FIG. 15 during reading of data from the memory.

DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

[0036]FIG. 1 provides a diagram of the major components of anon-volatile memory system that are relevant to the present invention. Acontroller 11 communicates with a host system over lines 13. Thecontroller 11, illustrated to occupy one integrated circuit chip,communicates over lines 15 to one or more non-volatile memory cellarrays, three arrays 17, 19 and 21 being illustrated, each of the arraysusually formed on one or more separate integrated circuit chips. Theillustrated controller is usually contained on a single integratedcircuit chip, either without a flash EEPROM array (the example shown) orwith a memory cell array. Even if a memory cell array is included on thecontroller circuit chip, an additional one or more chips that eachcontain only a memory array and associated circuitry will often beincluded in the system.

[0037] User data is transferred between the controller 11 and multiplememory arrays 17, 19 and 21, in this example, over the lines 15. Thememory arrays are individually addressed by the controller.Specifically, the data bus within the lines 15 can be one byte wide. Thememory system shown in FIG. 1 can be embedded as part of a host systemor packaged into a card, such as a card following one of the cardstandards previously mentioned. In the case of a card, the lines 13terminate in external terminals on the card for mating with acomplementary socket within a host system. Although use of onecontroller chip and multiple memory chips is typical, the trend is, ofcourse, to use fewer separate chips for such a system by combining theircircuits. An example capacity of one of the illustrated memory chips is256 Mbits, thus requiring only two such memory chips, plus thecontroller chip, to form a non-volatile memory system having a datacapacity of 64 megabytes. Use of a single smaller capacity memory chipresults in a memory system of lesser capacity, an 8 megabyte systembeing a marketable example. Conversely, use of memory chips with ahigher bit storage density and/or use of more memory array chips in asystem will result in a higher capacity memory. Such memory systems upto 1.3 gigabyte and more are possible.

[0038] The controller 11 includes a micro-processor or micro-controller23 connected through controller interface logic 25 to internal memoriesand interfaces with external components. A program memory 27 stores thefirmware and software accessed by the micro-controller 23 to control thememory system operation to read data from the connected memory array(s)and transmit that data to the host, to write data from the host to thememory chip(s), and to carry out numerous other monitoring andcontrolling functions. The memory 27 can be a volatile re-programmablerandom-access-memory (RAM), a non-volatile memory that is notre-programmable (ROM), a one-time programmable memory (OTP) or are-programmable flash EEPROM system. If the memory 27 isre-programmable, the controller can be configured to allow the hostsystem to program it. A random-access-memory (RAM) 29 is used to store,among other data, data from tables read from the non-volatile memorythat are accessed during reading and writing operations.

[0039] A logic circuit 31 interfaces with the host communication lines13, while another logic circuit 33 interfaces with the memory array(s)through the lines 15. Another memory 35 is used as a buffer totemporarily store user data being transferred between the host systemand non-volatile memory. The memories in the controller are usuallyvolatile, since memories with fast access and other characteristicsdesired for efficient controller access have that characteristic, andmay be combined physically into a single memory. A dedicated circuit 36accesses the streaming user data being transferred to the memory andinserts dummy bytes into the data stream in order to avoid writing validuser data to memory cells in bad columns. A dedicated processing circuit37 also accesses the streaming user data being transferred between thecontroller and flash interfaces 25 and 33 for generating an ECC, orother type of redundancy code, based upon the user data. When user datais being transferred into the non-volatile memory, the generated ECC isappended onto the user data and simultaneously written into the samephysical block of the non-volatile memory as part of the same sector asthe user data. The circuits 36 and 37 are described further below, withrespect to FIGS. 6A-C.

[0040] The non-volatile memory chip 17 includes a logic circuit 39 forinterfacing with the controller through the lines 15. Additionalcomponents of the memory chip are not shown for simplicity inexplanation. The purpose of the logic circuit 39 is to generate signalsin separate buses and control lines. Various control signals areprovided in lines 41 and a power supply 43 to the memory array circuitsis also controlled through the interface 39. A data bus 45 carries userdata being programmed into or read from the non-volatile memory, and anaddress bus 47 carries the addresses of the portion of the memory beingaccessed for reading user data, writing user data or erasing blocks ofmemory cells.

[0041] The floating gate memory cell array of a single non-volatilememory chip is itself divided into a number of units that each have itsown set of supporting circuits for addressing, decoding, reading and thelike. In this example, eight such array units 0-7, denoted by referencenumbers 51-58, are illustrated. Physically, as an example, the memoryarray on a single chip is divided into quadrants, each quadrantincluding two units that are in part connected together and share acommon word line decoding circuit (y-decode), such as a y-decoder 61between memory cell units 4 (55) and 5 (56). This memory architecture issimilar to that described in aforementioned U.S. Pat. No. 5,890,192,except there are eight units instead of the four units (quads)illustrated in that patent.

[0042] Each of the array units has a bit line decoder (x-decode), suchas x-decoder 63 connected to the array unit 5 (56), through which userdata is read. FIG. 2 is an expanded view of the array unit 5 and itsdecoders 61 and 63 that respond to addresses on the address bus 47.Connected to the decoder 63 is a circuit 65 that contains senseamplifiers for reading data, a register for storing data beingprogrammed, comparitors used during programming to determine whetheraddressed cells in the unit 5 have been programmed to the desired stateand during reading to determine the states of the cells being read, andcontrol logic to carry out these functions. Two registers 67 and 69 areconnected for parallel transfer of user data between them during reading(from 67 to 69) and programming (from 69 to 67). User data istransferred from the data bus 45 and the register 69, one byte at atime, during writing and in the other direction during reading. Each ofthe other seven array units are similarly connected.

[0043] Referring specifically to FIG. 2, a portion of an example memorycell array is generally described with respect to the array unit 5. Eachrow of cells has its own conductive word line (WL) connected through thedecoder 61 to corresponding word lines of the adjacent array unit 4.Each of two partial rows 70 and 76 of respective floating gate memorycells 71-75 and 77-81, for example, has its own respective word line 83and 85. A word line is connected to a gate of each of the cells in asingle row, the connected gate being a select gate in a memory cellhaving a split channel type of structure. Other memory cell structurescan be used instead, each having at least one electrically floating gateupon which a level of stored charge is a measure of the state of thecell. A conductive erase line is provided between every other row ofmemory cells, the line 87 being connected to erase gates of each of thememory cells of each of the rows 70 and 76. Alternate structures do noterase the floating gates to a separate erase gate but rather erase to aregion of the substrate such as the cell source diffusions. Bit lines(BL) extend in an orthogonal direction to the word lines, one bit linebetween each column of array cells, and are connected to the decoder 63.Each bit line is connected to the source and drain diffusions of each ofthe cells of the columns on either side of the bit line. Detailedexamples of suitable memory arrays are described in the U.S. patentslisted in the Background section above but other existing and proposedstructures can alternatively be employed in implementations of thepresent invention.

[0044] A block of cells is formed, in the array example being described,from each pair of rows that surround an erase gate, such as the rows 70and 76 of the array unit 5 (FIG. 2) on either side of the erase gate 87,when operating each floating gate in four defined threshold voltagestates in order to store two bits of data per floating gate. This isillustrated more generally in FIG. 3A, where the block formed of rows 70and 76 contains a sector's worth of data from the host, plus someoverhead information about that data. In a very specific example, theindividual blocks are sized to have a capacity of 528 bytes in order tostore one 512 byte sector of data, some overhead information about thedata and provide some spare bytes, as illustrated in FIG. 4. When eachfloating gate is capable of storing two bits of data, as is the caseillustrated in FIG. 3A, each block contains 264 floating gates, or 132in each of the block's two rows. If the memory cell structure has onefloating gate per cell formed between source and- drain diffusions, then132 cells are included in each row of each one of the units. But if thecells each have two floating gates, then only 66 cells are necessary ineach row.

[0045] If, rather than storing two bits of data per floating gate, onlyone bit is stored per floating gate, twice the number of cells areneeded for each operable block. This can be accomplished, in effect, bydoubling the length of each row in a manner illustrated in FIG. 3B. Therows 70 and 76 are extended to include the respective rows 70′ and 76′of the adjacent unit 4 with which unit 5 is paired. The word line of row70′ of the unit 4 is connected through the decoder 61 to the word lineof the row 70 of the unit 5, and the word line of the row 76 isconnected to the row 76′ of unit 5. When a given row's word line isaddressed, both components of the word line, in adjacent units 4 and 5,are addressed together. The common erase gate for the rows 70 and 76 isalso connected through the decoder 61 to a common erase gate for therows 70′ and 76′, in order to cause all of the cells formed of theenlarged block of cells of the rows 70, 70′, 76 and 76′ to be erasedtogether. Therefore, when operating the memory array floating gates withonly two threshold states, the eight units of the array being describedare reduced to four operating quadrants that are each formed of adjacentpairs of units (0-1, 2-3, 4-5 and 6-7). This allows the memory array ofFIG. 1 to be operated in either of two or four states, according to acommand set by the manufacturer, when a proper set of referencethreshold levels are also set for program-verify and reading in eachunit's circuits 65.

[0046] As an alternative to the two state configuration shown in FIG.3B, a single block may be formed of rows that are all within a singleunit, such as four rows that are contiguous with each other.

[0047] Data is preferably transferred in chunks between the controllerbuffer memory 35 (FIG. 1) and an addressed block of the array memorycells. In a specific example, each chunk contains 66 bytes of data. Achunk of data is programmed to the cells in an addressed block, one timein parallel. When operating in four states, chunks of data 91-94 arestored in one row of a typical block (FIG. 3A), and chunks of data97-100 are stored in the second row of cells of that block. When thememory is operated in two states, individual chunks of data 91′-94′ arestored one of the extended rows of cells (FIG. 3B), and chunks 97′-100′in the other row of the expanded block of cells that extends across twoadjacent units.

[0048] Rather than transferring a chunk of user data in parallel betweenthe buffer memory 35 and one of the memory arrays, a data bus within thelines 15 is designed to carry only a few bits data in parallel, one bytein a specific example. This reduces the number of lines 15 that arenecessary and, more importantly, reduces the number of user data padsthat need to be included on each of the memory array chips in thesystem. With one byte being transferred at a time, there thus needs tobe 66 such transfers for a each chunk. These byte wide user datatransfers extend between the buffer memory 35 and, through interfacecircuits 33 and 39, the memory cell array master register 69, if a blockwithin the array unit 5 is being addressed, or another of the arraymaster registers associated with another unit if a block within thatother unit is being addressed.

[0049] During programming, bytes of a sector of user data received fromthe host system are successively transferred into the memory arraymaster register one at a time until a chunk of data has beenaccumulated, and then the chunk is transferred in parallel to the slaveregister (such as register 67 for the array unit 5). As the chunk isbeing transferred out of the master register and into the slaveregister, bytes of the next chunk of the sector are being transferredinto the master register. Chunks are transferred in parallel, one at atime from the slave register, to programming and verifying circuits(circuit 65 for the array unit 5), which causes the a number of memorycells of an addressed block of the associated array unit to beprogrammed to their respective target threshold levels. The loading ofone chunk into the master register preferably overlaps programming ofthe previous chunk of data from the slave register by the programmingand verifying circuits.

[0050] During reading, the process is reversed, memory cell thresholdvalues representative of a chunk of data being read one at a time fromone of the array units into its reading circuit (circuit 65 for thearray unit 5) where the stored values are converted into data bits.Those successive chunks of data are then transferred one at a time intothe slave register for the array unit, thereafter in parallel to themaster register, and then one byte at a time over the lines 15 to thedata buffer 35 of the controller for transfer to the host system. As achunk is being transferred out of the master register and to thecontroller, a new chunk of data is being transferred into the slaveregister from the read circuits.

[0051] Rather than completing the transfer of chunks of data of onesector before commencing another, it is preferred to alternatelytransfer chunks of multiple sectors between the buffer memory 35 anddifferent ones of the array units 0-7. This is illustrated in FIG. 5,where data from four sectors is transferred in a common data stream 101.Memory array units A, B, C and D can be any four of the array units 0-7of the system of FIG. 1. Chunks of data A0+, B0+, C0+ and D0+ are shownto be transferred in the data stream 101 from four respective datasectors 103, 105, 107 and 109 in the buffer memory 35 to the fourdifferent memory units A-D. The memory system can, of course, bealternatively designed to transfer fewer or more data sectors togetherto an equal number of memory cell units. For simplicity in illustration,each sector is shown in FIG. 5 to contain only four chunks (chunks A0,A1, A2 and A3 for the sector 103, for example), where the system of FIG.1 has been described above to transfer eight chunks of data for eachsector. The principle of operation is the same.

[0052] As illustrated in FIG. 5, the data stream 101, being transferredin the data bus of the lines 15, is formed from successive bytes of afirst chunk A0 of the buffered data sector 103, followed by a firstchunk B0 of the sector 105, then a first chunk C0 of the sector 107, andthen a first chunk D0 of the sector 109. These chunks are initiallystored in the master registers of the respective units to which the dataare being written, transferred in parallel to the slave registers andthen written in parallel into respective blocks 111, 113, 115 and 117 ofthe four memory cell units A-D. All of the chunks A0, B0, C0 and D0 areprogrammed at the same time into respective memory units A-D, followedby chunks A1, B1, C1 and D1, and so forth. The blocks where chunks ofdata are written either share a common word line 119 or have the samevoltage impressed on the word lines of the addressed blocks. As theinitial chunks of data are being transferred through the registers andprogrammed into the units, subsequent chunks A1, B1, C1 and D1 of thefour data sectors are being transferred to the registers of the units.This is followed, in the data stream 101, by another chunk from each ofthe data sectors 103, 105, 107 and 109, and so forth, until all of thechunks of each of the four buffered data sectors has been programmedinto the memory units A-D.

[0053] This programming technique has an advantage that multiple chunksof user data may be simultaneously programmed into different units ofthe memory on a single chip, or, alternatively, into different unitsspread among two or more memory chips. This is preferred to thesimultaneous programming of multiple sectors of data into a common blockof one unit since the cells being programmed are, by the technique ofFIG. 5, physically and electrically separated further from each other.This separation reduces incidents of disturbing the charge on floatinggates not intended to be altered. It also spreads out the power supplyrequirements on the chip. The technique of FIG. 5, which operates withmemory blocks formed of contiguously positioned memory cells, is alsopreferred to defining memory blocks in segments across multiple memoryunits in order to simultaneously program multiple segments of the blockwithout causing disturbs and having increased power requirements.

[0054] The technique of FIG. 5 has been described with the assumptionthat all of the multiple data sectors, four in that example, are writtenin full into the controller buffer 35 before transfer of their data inchunks to the flash memory is commenced. But this transfer can becommenced earlier if all four of the data sectors are beingsimultaneously written into the buffer memory 35, as can be done withthe higher data transfer rates between the controller 11 and the hostsystem. Alternatively, some additional parallelism can be included byloading the four data sectors 103, 105, 107 and 109 into the buffermemory 35 in a time shifted manner, such as by loading data of the chunkB0 from the sector 105 after the chunk of data A0 has been written andwhile it is being read out into the data stream 101, and so forth downthe line through the data sectors 107 and 109. The data sectors are thenloaded into the buffer 35 with a time shift of that necessary to loadone chunk of such data. The generation of chunks of data within thestream 101 then needs to wait for only the first chunk of each datasector to be loaded into the buffer 35.

[0055]FIG. 6A is a block diagram of a circuit that may be included aspart of the controller 11 of the FIG. 1 memory system in the path ofuser data being transferred in a stream between the interfaces 25 and 33during both programming and reading. These circuits participate incarrying out the method described with respect to FIG. 5, and alsogenerates and inserts into the data stream an ECC from the data beingprogrammed (ECC generation 37) and make use of the BCPs to avoid writinguser data to memory cells within bad columns (BCP processing 36). Themicro-controller 23 addresses the buffer memory 109, one byte at a timein a predefined sequence. That sequence is altered by circuits 104 and106 in response to a signal in a line 108 that indicates a byte addresswithin the flash memory includes a bad column. A multiplexer 121operates to supply bytes of data from one of sectors A, B, C or D at atime, in response to a control signal in lines 123, as one input toanother multiplexer 125. Chunks of data are preferably read from thedata sectors stored in the buffer memory 35 in the manner describedabove with respect to FIG. 5. A control signal 123 causes themultiplexer 121 to switch from one data sector to the next, in sequence,each time that a chunk's worth of data bytes has been read from thebuffer 35.

[0056] The stream of user data bytes at the output of the multiplexer121 is applied as one input to a multiplexer 125. The multiplexer 125normally passes that data stream from lines 114 through its output toone input of another multiplexer 112 An exception is when all the userdata bytes of a sector have been passed out of the buffer 35, at whichtime a final series of bytes containing an ECC code for the sector ofuser data is added onto the end of user data through lines 116. Theoutput of the multiplexer 125 provides the byte wide stream of user dataand overhead information of the data for writing into physical blocks ofthe non-volatile memory.

[0057] That stream of user data is normally passed, one byte at a time,through the multiplexer 112 to the flash memory An exception to this iswhen a BCP hit signal in line 108 is active, indicating that the byte ofuser data is being directed to a location of the flash memory thatincludes a bad column. In that case, the address applied to the buffer35 is not incremented by the micro-controller 23 but rather a fill bytecontained in a register 110 is inserted into the data stream instead ofthe usual byte of user data. The next byte of user data from the buffer35 is delayed until the next transfer cycle when the BCP hit signal isinactive. The bits of the fill byte within the register 110 can bewritten by the micro-controller 23. The output of the multiplexer 112provides the byte wide stream of user data and overhead information ofthe data that is written into physical blocks of the non-volatilememory, as well as possible fill bytes to avoid the effects of badcolumns. The memory system being described can operate without thecircuit of FIG. 6A, or with one of the BCP processing 36 or the ECCgeneration 37 but not both, but it is preferred to include bothfunctions as shown.

[0058] Referring to FIG. 6B, additional details of the ECC generationunit 37 of FIG. 6A are given. Even though four sectors of data arealternately being transferred in a byte wide data stream from the outputof the multiplexer 112 into a single ECC generator circuit 127, the ECCgenerator 37 is able to generate an ECC separately for each data sectorand append the generated code to the end of each sector's data in thelast chunk that is sent to the non-volatile memory array for storage.This is done by use of separate registers 133, 134, 135 and 136 for eachof the four data sectors being sent to the memory array at one time. Aresulting generation is stored in one of these registers by ade-multiplexer 131 or some other effective switching logic. Similarly,the contents of an appropriate one of the registers is connected througha multiplexer 139 as an input to the ECC generation circuit 127. Aftereach byte of data is input to the ECC generation circuit 127 from thebuffer 35, that circuit uses an intermediate result of an ECC generationfor the same respective data sector A, B, C or D that is stored in therespective one of the registers 133-135, along with the new byte ofdata, to generate a new intermediate result that is stored back intothat same register. This use of the registers is controlled by theirinput de-multiplexer 131 and output multiplexer 139, which are caused bythe control signal in lines 123 to step in sequence between registers insynchronism with the data selecting multiplexer 121. A standard ECCalgorithm, such as the Reed-Solomon code, is implemented by thegeneration circuit 127.

[0059] In order to pause the ECC generator 127 when fill bytes are beinginserted into the data stream by the BCP processing, the generator 127is disabled by a combination of the BCP hit signal in the line 108 beingactive and a signal in the line 116 indicating that the data at theoutput of the muliplexer 121 is valid. A logic circuit 138 combines thesignals 108 and 116 in order to generate a disable signal for the ECCgenerator 127.

[0060] After the ECC generator has received the last byte of data of asector being stored, the final result is inserted in the last chunk ofthe data stream by operating the multiplexer 125, in response to acontrol signal in a line 129, to switch from receiving the input fromthe multiplexer 121 to receiving the result from the ECC generationcircuit 127. The ECC is then stored in the same block of the memoryarray as the sector of user data from which it was generated. At the endof generating and inserting ECCs into the data stream for each of agroup of four sectors of data, the registers 133-136 are reset by asignal in a line 141.

[0061] Referring to FIG. 6C, additional details of the BCP processing 36are given. A number of bad column pointers (BCPs) for each of the memorysectors being programmed at a particular time are loaded by themicro-controller 23 into registers 118 from a reserved block of theflash memory (described hereinafter with respect to FIG. 11). Four BCPs0-3 are stored for each of the memory units to which the stream 101(FIG. 5) is being directed. That is, all four BCPs 0-3 for each of thesefour units have been written by the micro-controller 23 from thereserved block into four planes (sets) of registers 118, each set havingfour registers. Each of the planes of registers 118 shown in FIG. 6Ccontains BCPs 0-3 for a different unit. A control signal in a line 123switches between these register planes in order that the BCPs for theunit for which a chunk of data currently in the stream of data from thebuffer 35 through the multiplexer 121 is destined. A multiplexer 120outputs one of the register values to one input of a comparitor 132,whose output is the line 108 that contains the BCP hit signal. One offour registers 122 is also included for each of the units for to whichdata of the current stream will be written. Each of the registers 122begins with a count that causes the multiplexer 120 to select BCP0 forits unit. Each time a BCP hit signal in line 108 occurs, circuits 124and 126 causes the BCP count of the appropriate register 122 toincrement by one.

[0062] A second input of the comparitor 132 is formed from the contentsof registers 128 and 130. These registers contain the current physicalmemory location for storage of the chunk and byte, respectively, towhich the byte of data at the output of the multiplexer 121 is destinedfor storage within the flash memory. These registers are loaded by themicro-controller 23. Their combined physical memory byte address is thencompared by the comparitor 132 with that of one of the BCPs from theregisters 118 of the destination memory unit for that data byte. Whenthe first byte of user data for a given unit is present at the output ofthe multiplexer 121, its physical memory address from registers 128 and130 is compared with BCP0 of the one of the registers 118 for that unit.If there is a positive comparison, then the BCP hit signal in line 108becomes active. This results in the current byte of user data of ECC tobe held and the fill byte from the register 110 inserted instead. Thefill byte will then be written to the byte of memory cells including thebad column, instead of the current data byte.

[0063] The BCP hit signal also causes the count in the one of theregisters 122 for that unit to switch the multiplexer 120 to select thenext BCP1 for that memory unit. The BCP processor is then ready for thisprocess to again take place when the physical memory byte locationmatches that of the next in order BCP for that unit. A stalled data byteis written during the next cycle, assuming this next physical addresscomparison does not result in another BCP hit, in which case the databyte is stalled yet another cycle. This process continues until the datastream formed of the current four sectors of data has been transferredto the flash memory, after which the process is repeated for differentmemory locations stored in the registers 128 and 130 in which the newsectors of data are to be written, and possibly changing the BCPs storedin one or more of the planes of registers 118 if the new data is to bewritten into one or more different memory units than before.

[0064] Referring again to FIG. 4, the specification of a block of thememory array that stores a sector of data and the ECC is given. Asadditional overhead information, a first byte 145 provides basicinformation of how the remaining data is written. This can include, forexample, one or two bits that designate the relative values or polaritywith which the stored data needs to be read. This is often changed fromblock to block and over time, in order to even the wear caused byprogramming the cells of a block to one of the programmed states fromtheir erased state (which can also be one of the programmed states).This is described further below with respect to FIGS. 15-17. A nextcomponent is data 147 which is, for what has been described so far, onesector of user data supplied by the host system for storage in thenon-volatile memory system. The component 149 is the ECC was generatedfrom the user data 147 during programming, in the manner describedabove. The number of cells in the individual blocks is chosen to leave agroup 151 of cells, capable of storing 8 bytes in this example, asspares. These spares are shown in FIG. 4 to be at the end of the blockwhen written or read, which is the case if there are no defects in therest of the block which require use of some or all of these spares. Thenumber of spare bytes 151 will be less than what is shown in FIG. 4 bythe number of fill bytes that are inserted into the user data 147 by theBCP processing 36 to avoid bad columns. The insertion of a fill byteinto the user data 147 causes subsequent bytes to be delayed by onebyte, or moved to the right in the sector data diagram of FIG. 4, thusdiminishing the number of spare bytes 151 at the end of the data streamby one.

[0065] The BCP processing and ECC generation circuits of FIGS. 6A-C areoperated in a reverse manner when sectors of data are being read fromthe flash memory, except that an entire sector of data is read beforedata from another sector is read. The ECC generation circuits 37calculate an ECC from the user data in the sectors as that data arepassed one byte at a time through the circuit of FIG. 6A from the flashmemory to the data buffer 35, in the same manner as described above forwriting data. The calculated ECC is then compared with the ECC bytesstored as part of the sector in order to determine whether the sector'suser data is valid or not. The BCP processing identifies bytes in theread stream of data that are fill bytes by comparing the physical memorylocation from which each byte is read with the BCPs for the memory unitis which the sector of data is stored. Those bytes are then eliminatedfrom the read data stream before writing the sector of data into thebuffer memory 35.

[0066] Referring to FIG. 7, the programming of data described in FIGS.4-6 is summarized. The dashed line represents the order of chunks ofdata appearing in the data stream 101 generated by the system of FIGS.1-3 for four data sectors A-D, where each sector is transferred in 8chunks. As can be seen, the first chunks of sectors A, B, C and Dappear, in that order, followed by the second chunks of sectors A, B, Cand D, and so forth, until the eighth and final chunks of data sectorsA, B, C and D are include. Thereafter, the process is likely repeatedwith four different sectors of data.

[0067] It will be noted that the overhead information that is stored ina block along with a sector of data is limited to information about thedata itself and does not include physical overhead information about theblock or its operation. Prior memory systems, particularly those whichemulate a disk drive by storing 512 byte sectors of user data, have alsostored, in the individual user data blocks, information about theblock's characteristics in terms of experience cycles, numbers of pulsesor voltages required to program or erase the block of cells, defectswithin the block, and like information about the storage media, inaddition to an ECC generated from the data stored in the block. As partof the present invention, however, this type of information about thephysical block is stored in another block. As a specific example,individual block overhead records containing such information of a largenumber of blocks are stored in other memory blocks dedicated to suchoverhead information and which do not contain user data. Suchinformation of the memory array that affects a number of blocks, such asthe identification of bad columns, is stored in yet other memory blocksin order to minimize the memory space that is required for suchinformation. In either case, the overhead information for a given blockis read from these other blocks as part of the process of accessing thegiven block to either read data from it or program data into it. Theblock overhead information is usually read by the controller prior toaccessing the block to read or write user data.

[0068]FIG. 8 illustrates a few such block overhead data records 151-157that are stored together in a single memory array block in the format ofFIG. 4. That is, the multiple overhead records of other blocks form thedata 147 of a sector of overhead data that includes the flag byte 145and ECC bytes 149 generated from the overhead data 147. The block inwhich the overhead data sector is stored also includes the spare bytes151 of cells for replacing any cells within a defective column. Thissector of overhead data and the block in which it is stored have thesame characteristics, and are written and read the same, as describedabove for user data sectors. The primary difference is the nature of thedata 147. In a specific example, each such overhead record contains fourbytes of data, resulting in 128 such records being combined togetherinto each defined sector of overhead data.

[0069]FIG. 9 illustrates example contents of a block overhead record fora good user data block. Byte 0 contains flags including an indicationthat the user data block is a good one. Byte 1 specifies a voltage forerasing the user data block, and byte 2 a programming voltage. These twovoltages are updated over the life of the memory system by thecontroller in response to the subject user data block requiring anincreasing number of erasing or programming pulses to reach the desiredrespective erase and programmed states. The controller uses thisinformation when erasing or programming, respectively, the subject userdata block for which the record 151 contains overhead data. Byte 3 ofthe overhead data record indicates the extent of use of the subject userdata block, either as an experience count that is updated each time thesubject user data block is erased and reprogrammed or as thecharacteristics of one or more tracking cells that are put through thesame number of erase and reprogramming cycles as their correspondinguser data block. The use of such tracking cells, preferably associatedwith each of at least the blocks of the memory array designated to storeuser data, is described in aforementioned U.S. patent application Ser.No. 08/910,947. Data in byte 3 is periodically rewritten as the numberof cycles of use increase or as the characteristics a tracking cellchange significantly. The value of byte 3 is used to determine when theassociated user data block needs to be retired from service. Use oftracking cells, rather than an experience count, has a significantadvantage that the overhead sector data need be rewritten far lessfrequently, as little as just a few times over the lifetime of thememory array, rather than after each erase/program cycle.

[0070]FIG. 10 illustrates an overhead record for a user data block thathas exceeded its useful lifetime or otherwise has been determined by thecontroller to be a defective block. The flag byte indicates that theblock is defective and a spare block has been assigned to take itsplace. (Not to be confused with the flag byte 145 (FIG. 4) that is atthe beginning of the individual data sectors.) The other three bytes ofthe record specify, as part of the current user data block overheaddata, the spare block's address. Thus, the three bytes used for a goodblock to specify its operating parameters (FIG. 9) are efficiently usedfor this other purpose for a defective block. This minimizes the numberof bytes required for the overhead data. When the controller reads thisrecord as part of the process of accessing its user data block, it isquickly determines that the addressed block is defective and the addressof the spare block provided in the record of FIG. 10 is then used by thecontroller to address and access the spare user data block.

[0071] This arrangement of blocks thus requires that a number of spareblocks be provided in addition to the number of user data blocksnecessary to fill the address space specified for the memory array. Theoverhead records for these blocks designate their status as spares inthe flag byte, and whether they are good or defective spare blocks. If agood spare block, the record contains the same bytes 1-3 as the recordof FIG. 9. If a defective spare block, the bytes 1-3 need not containany overhead information of the block since it will never be used.

[0072] Any defects of the memory cell array that affect many blocks ofcells, such as defective columns as can occur when a bit line has ashort circuit to some other element, are stored in other blocks in orderto compact the block overhead records. An example shown in FIG. 11 is abad column pointer (BCP) table stored as data in a block having the dataformat of FIG. 4. Such a table is made during the testing phase of thememory system manufacturing process. In this example, up to four BCPs oftwo bytes each may be stored for each of the memory cell units 0-7.Since eight spare bytes of cells are included as spares 151 in a typicaluser data block (FIG. 4), two bytes of cells may be skipped in a singleblock in response to the controller reading one BCP from the table ofFIG. 11. If there are more than four bad columns in a unit, that unit isidentified as defective and not used. The memory system may operate solong as at least one unit remains usable. Alternatively, since thecolumn lines of each unit may be segmented, some number of BCPs, such astwo, may be stored for each segment or for individual groups ofsegments. It does, of course, take more memory to store an expanded BCPtable.

[0073] During operation of the memory, the BCP table of FIG. 11 and asmany overhead data blocks of FIGS. 8-10 as space allows, are read fromthe memory blocks by the controller into its RAM 29 (FIG. 1) withoutdeleting that overhead data from the non-volatile memory. This is doneupon initialization of the memory system and, if there is not enoughroom in the RAM 29 for all of the block overhead sectors of data, thosethat are less frequently accessed by the controller are deleted from theRAM 29 in favor of those that need to be accessed. Since this data canbe read much faster by the controller from its own RAM 29 than from anon-volatile memory block, the “caching” of such data in the controllermemory speeds up the process of accessing non-volatile memory blockscontaining user data since the overhead data must also be accessed.

[0074] The controller 11 may access a number of user data sectors inresponse to receipt from a host system of a command containing anaddress, such as a in a cylinder/head/sector format, as in a disk drive,or as a logical block. The controller then calculates a logical addressof a beginning block corresponding to the beginning sector addressprovided by the host. The memory system address space for a given arraychip may be expressed as a continuum of logical block addresses (LBAs)that represent all available blocks on the chip in its good memory unitsfor storing user data. The block overhead records are logically arrangedin that same order. The controller then first reads the block overheadrecord in its RAM 29 that corresponds to the first data sector specifiedby the host while a physical address within the memory array of the userdata block is being calculated from its LBA. After accessing the firstuser data block and its overhead information, the logical block addressneed not be recalculated for each subsequent block that is addressed fordata of the particular file. A simple counter will step through theoverhead records that have been organized in the order of theirrespective user data blocks. If all of the necessary overhead recordsare not in the controller's RAM 29 at the time of this access, therequired overhead data sectors are preferably first read from thenon-volatile memory into the RAM 29, including those of substituteblocks whose addresses are contained in the initial overhead records.

[0075] One advantage of storing the block overhead data in recordsseparate from the block is the reduced number of times that such recordsmust be rewritten. In the present embodiment, the overhead blocks ofdata need not be rewritten frequently, perhaps only two or three timesduring the lifetime of the memory system and some times not at all. Anychange to the overhead information record for one block is held as longas possible in a controller memory before rewriting the overhead datasector in which record exists, and then it can be done in the backgroundwithout being part of an erase or programming cycle. But when theoverhead data of a block is stored in that block, the overhead data mustbe reprogrammed each time the block is erased. In the examples describedherein where only one sector of user data is stored in a block, overheaddata would have to be rewritten in one block every time a sector of userdata is written into that memory block. This can also require theoverhead information to be written twice into the same block, oncebefore writing the user data and once after doing so, in order tocompensate for effects due to programming adjacent cells, particularlywhen being programmed in multiple states where the tolerance of sucheffects is less.

[0076] Although the memory system examples being described store onlyone sector of user data in each of the individual blocks, variousaspects of the present invention apply equally well where two or moresectors of data, each with its flag and ECC bytes, are stored inindividual blocks of the memory cell array.

[0077]FIG. 12 provides an example utilization of the individual,blocksof a memory array chip having eight units of blocks. The same bootinformation is stored in a specified block of each of the units, usuallythe first block. Upon initialization of the system, the controllerfirmware causes the first block of unit 0 to be read but if that blockis not readable or its stored′ data corrupted, the first block of unit 1is accessed for reading, and so on, until valid memory system bootinformation is read by the controller. It is then stored in thecontroller RAM 29. There are also a number of reserved blocks, such asreserved blocks 0-7 being shown in FIG. 12, in which data desirable foroperation of the memory array chip are stored. A copy of the data ofeach of the reserved blocks is provided in a different memory unit thanthe primary copy as insurance against a defective unit. The data formatof each of the reserved blocks is that illustrated in FIG. 4.

[0078] Part of the boot information is a physical address of reservedblock 0 and its copy, which, in a specific example, contains the badcolumn pointers of FIG. 1i, an identity of any unusable unit(s), andphysical mapping characteristics including which of the blocks of goodunits are reserved, those designated for user data, those designated foroverhead data (“O.H. Data”) according to FIGS. 8-10, and thosedesignated as spare blocks. Reserved block 0 also contains read andwrite control parameters particular to its memory array chip.

[0079] The other reserved sectors contain information that is useful tothe controller to operate the memory array chip on which they arelocated, or useful to the entire memory system. Reserved block 1, forexample, can contain data of system parameters that appears on only thefirst logical memory array chip of the system, including a specificationof the data interface 13 of the controller with the host system. Anotherreserved block may contain manufacturing information, for example. Thedata stored in the boot information and reserved blocks are written aspart of the manufacturing process. This data may be made modifiableafter manufacture, either in part or in total.

[0080]FIG. 13 illustrates a desired step in the manufacturingconfiguration process. Data about each memory array chip is read by thecontroller 11 from the reserved sector 0 of each chip and assembled intoa common file illustrated in FIG. 14 that is then written back intoreserved sector 2 of the first logical chip 17 of the memory system.This process can be provided by firmware included in the program memory27 (FIG. 1) of the controller. Upon invoking this firmware routineduring the system configuration, data of the number of good user datablocks is read from each of the system's memory chips. That informationthen becomes a part of the system record of FIG. 14. Logical blockaddress (LBA) ranges are also added to that record, the first user blockof the first logical memory chip 17 being assigned an address of 0000.The ending LBA is also recorded as part of the record, being thebeginning LBA of zero plus the number of user data blocks on the firstchip. The logical memory space for the next memory chip 19 is noted byits number of good user data blocks and its ending LBA, which is onemore than the ending LBA of the first memory chip plus the number ofgood user data blocks in the second chip. This process continues untilthe table of FIG. 14 is completed for all the memory array chips.

[0081] In a specific example, the entry in the table of FIG. 14 for eachmemory chip in the system includes a one byte physical chip number andthree bytes for the ending LBA. This results in the merged system LBAtable being very short and quick to access. When accessing a particularuser data block of the system in response to an address from a hostsystem, as a particular example, the controller 11 first calculates acorresponding logical block address. That LBA is then compared with thetable (FIG. 14) in reserved block 2 of the first logical memory chip 17to determine on which of the chips the addressed block lies. The LBA isof the immediately preceding logical chip is then subtracted from thecalculated LBA. The physical block address on that chip is thencalculated by the controller reading the designated chip's reservedsector 1 information to shift this differential LBA into a correspondingblock of the designated chip that has been designated for user datastorage.

[0082] In addition to storing a record of the number of good units andsectors from which the merged table of FIG. 14 is formed, each of thememory array chips includes other information about its respective arraythat the controller uses to operate it. This other information can alsobe stored in the reserved sector 0 of each chip array. Another featureof the present invention is that multiple array chips that are usedtogether with a common controller in one system need not each have thesesame characteristics. The controller 11 preferably reads thecharacteristics stored in reserve sector 0 of an individual chip intoits RAM 29 before accessing that chip. This can be once when the memorysystem in initialized, provided the controller RAM 29 is large enough,and then accessed from RAM during operation of the memory. Alternately,this information can read from a particular chip just before it isaccessed each time and then stored in the RAM 29 only during the time ofsuch access. It is a significant advantage to be able to form a memorysystem with two or more memory chips that have different operatingcharacteristics. This greatly simplifies the manufacturing process sincememory chips from different process batches, different foundries,different manufacturers, and so on, can be combined to operate welltogether. Testing of the chips in order to batch together those havingthe same values of these characteristics is no longer necessary.

[0083] These characteristics that can be different among memory chipsinclude various optimal voltages and timing, as well as minimum andmaximum permitted values, for use to program, read, erase, scrub andrefresh the chip. The optimal number of programming pulses, theirdurations, frequency and magnitudes, as well as operatingcharacteristics of voltage pumps on the chip, including ranges ofmaximum and minimum values can also be included. The number of sectorsper unit and other information needed by the controller to translate alogical block address to a physical one within the particular chip arealso stored. Pointers to spare units and blocks within units may also beincluded. Other information includes pointers to the physical locationsof the reserved sectors so that they need not all be stored in the sameblocks in each memory chip of a system. The number of user data blocksfor which data is included in each of the overhead blocks (FIG. 8) canalso be stored. Inclusion of the bad column pointers (BCPs) in reservedsector 0 has already been described. A duplicate set of certainoperating parameters can be also be included for different chip supplyvoltages, such as 3 and 5 volts. The controller can then tailor itsaccess to match the characteristics of each chip in the system, whichneed not, as a result, be made the same.

[0084] The flag byte 145 (FIG. 4) of a sector of data is described withrespect to FIG. 15. In a specific example, the most significant bits 0and 1 of that byte give a factor by which all the following bits in thatsector are transformed. These two bits are preferably randomly selectedduring the writing process for each sector of data by themicro-controller 23 or fixed logic such as a state machine, prior userdata of the sector being read into the data stream 101 (FIG. 5).Alternatively, the transformation bits may be assigned by sequencingthrough all the possible combinations in order. These transformationbits are positioned at the beginning of the remaining flag bits, whichare in turn inserted into the stream of data prior to the user data andECC bytes that form the sector being stored. The purpose of sotransforming the data stored in the flash memory is to cause the memorycells to be programmed to different states for the same data. Thisprevents uneven wear of the various blocks of the memory that can occurover the life of the memory, such as when substantially the same datafile is repetitively written to the same group of blocks. When the datasector is read, these transform bits are first read and then used totransform the raw data read from the memory back to the data that wasoriginally received by the memory for storage. Two transform bits areused for a four state memory operation, more being used when the numberof storage states of the individual memory cells is made higher. If thememory is operating in a two state mode, however, only onetransformation bit is used.

[0085]FIG. 16 illustrates an example process of transforming receiveddata before being stored in the flash memory. This process can becarried out primarily by the micro-controller 23 under the control ofits firmware, or can be executed by the use of adder circuits andrelated digital elements. Each byte of data from the buffer 35 has itsbits separated into four pairs, and each bit pair is combined inrespective adders 175-178 with the two bit transform (for a four stateflash memory) that is held at 181. The transformed bit pairs are thenrecombined into a single byte that is applied to one input of amultiplexer 183. These transform bits are also used to transform, inadders 185-187, six fill bits indicated at 189. The transform bitsthemselves are combined with the transformed fill bit pairs to form thefirst byte 145 (FIG. 4) of a data sector, that is applied to a secondinput of the multiplexer 183. A control signal in a line 191 thenswitches the multiplexer 183 to output in circuits 193 successive bytesof data wherein the first byte of the sector is in the form of FIG. 15received through the multiplexer 1 input and subsequent bytes oftransformed data are received through the 0 input until the entiresector of data is transformed and sent to the flash memory. It is thistransformed data, rather that the raw data received from the host, thatis subjected to the BCP processing and ECC generation of FIG. 6A.

[0086] An inverse transform takes place when the sector of data is readfrom the flash memory, as illustrated in FIG. 17. Four subtractionelements 195-198 receive respective pairs of bits of a data bytereceived in the path 194 form the flash memory. The two transform bitsfor the particular data sector are stored at 199, which may be aregister. The transform bits, which are the first two bits of the firstbyte received for the sector, are stored at 199 in response to receivingthe first byte control signal in the line 191. These two bits arecombined with each of the received byte pairs in the subtractingelements 195-198 in a manner to transform them back to the raw receiveddata. The inverse transform of the sector data after the first two bitscommences immediately after those transform bits are read. There iscertainly no need to read the transform bits in a separate operation.The remaining 6 bits of the first byte are immediately transformed byits three pairs passing through three of the subtracting elements195-198 and then being stored at 201 in response to the control signalin the line 191. As subsequent bytes of the data sector are received,one at a time, the contents at 199 and 201 remain unchanged while theinverse transform of the data bytes is made and the result output to thebuffer memory 35 over a circuit 203. This output data is then the sameas that which was initially received by the memory system and stored ina transformed form.

[0087] The least significant bits 4-7 of the flag byte 145 illustratedin FIG. 15 are used by the controller to determine whether a block iserased or not. In one example of a memory system, the erased state isalso one of the programmed states, so some care needs to be taken todistinguish between them. When necessary to determine whether a block inthis type of system is erased from the data in the block, therefore, thesampling of one bit of data, for example, cannot itself provide theanswer. Therefore, in one example, fill bits 189 (FIG. 16) that arewritten into the first byte of a data sector are either a fixed knownpattern or made to have a pattern that assures those bits are not allequal to the value of the erased state. The controller reads any datasector in the block and looks for whether the pre-erase bits 173 storedin the corresponding cells of the block either are the fixed values ofthe fill bits 189 or, if there is no known pattern, at least one of them(and preferably more) is in a programmed state that is not the erasedstate. As a further check, it may be determined whether an ECC code ispresent in the data sector. If either or both of these conditions aredetermined to exist, it is concluded that the block is not erased. Butif the bit values of data read from the flag byte bit locations 4-7and/or the ECC all have the same value equal to that of the erasedstate, this indicates that the block is in its erased state.

[0088] Although specific examples of various aspects of the presentinvention have been described, it is understood that the presentinvention is entitled to protection within the scope of the appendedclaims.

1. A method of operating a re-programmable non-volatile memory systemhaving its memory cells organized into distinct blocks of simultaneouslyerasable cells, comprising: designating a first group of said blocks forstoring user data and a second group of said blocks for storinginformation of the characteristics of said first group of blocks,storing, in individual ones of the first group of said blocks, user dataplus characteristics of the user data being written therein but notincluding characteristics of said first group of blocks, and storing, inindividual ones of the second group of said blocks, a plurality ofrecords of characteristics of individual ones of the first group ofblocks but without storing either user data or characteristics of theuser data into the second group of blocks. 2-66. (canceled)